The disease of whooping cough or pertussis is a result of infection by Bordetella pertussis, and is a serious and debilitating human disease particularly in young children. For the last fifty years the disease has been controlled through large-scale immunization programmes. The current licensed vaccine in North America is a "whole cell" vaccine prepared by growing the organism in fermentors and then treating the resulting B.pertussis cells with chemical agents, such as formaldehyde, to kill the organism and inactivate toxic proteins. The cells are resuspended and then used directly or in combination with other antigens. This vaccine, although highly efficacious, has been associated with clinical symptoms that include fever, local reactions, high-pitched crying and convulsions. Despite the fact that there is no proven relation between these symptoms and the vaccine, there has been decreased public acceptance of this vaccine and in a number of countries, e.g. Japan, Sweden and the U.K., decreased immunization has led to outbreaks of the disease. The need for a more defined vaccine has been recognized and considerable effort has been directed by several manufacturers and researchers towards the development of an efficacious pertussis vaccine that consists of a small number of highly purified proteins. This vaccine has been termed a component vaccine.
This search has been hampered by a lack of information on the mechanism of pathogenesis of B.pertussis. Many virulence associated factors, such as pertussis toxin (PT), also known as lymphocytosis promoting factor (LPF), filamentous haemagglutinin (FHA), adenylate cyclase, lipopolysaccharide, agglutinogens and other outer membrane proteins have been suggested for inclusion in an "acellular" vaccine, which is less defined than the component vaccine. Much of the work on acellular vaccines has concentrated on a PT-based vaccine. Results of a recent clinical trial have indicated that a vaccine consisting entirely of PT-toxoid only partially protected children from the infection. A PT/FHA combination showed slightly higher efficacy but this was still lower than that obtained for the whole-cell vaccine.
One potential protective antigen is an outer membrane protein, with a molecular weight of approximately 69,000 daltons (pertactin) found on all virulent strains of B.pertussis. This protein is produced in relatively large amounts during the culture of the organism and can be purified from either the fermentation broth or from cell extracts. The present invention provides a novel method of effecting such purification.
The potential importance of this outer membrane protein for inclusion in a human vaccine against whooping cough was suggested from attempts to prepare a vaccine to protect pigs against B.bronchiseptica infection. Cell-surface extracts of B.bronchiseptica were used to immunize sows. Levels of antibody to a cell surface antigen with a molecular weight of 68,000 daltons correlated with protection of newborn piglets against infection. Similar antigens, with similar molecular weights, were detected in B.pertussis (approximately 69,000 daltons) and in B.parapertussis (approximately 70,000 daltons). Immunization with the protein obtained from B.pertussis protected mice against intracerebral challenges with live organisms and antibodies to the protein conferred passive protection to mice in this test. Both active and passive protection of mice in an aerosol challenge model have also been described.
The published procedures for purification of pertactin do not allow for the large-scale production of a highly purified, non-pyrogenic and stable antigen. One reported method (Canadian Patent No. 1,253,073) involves acid-glycine extraction of the cells, anion-exchange chromatography and preparative iso-electric focussing. However, the pertactin obtained has been reported to degrade into smaller fragments, to be sensitive to low pH and to have adenylate cyclase activity. For these reasons, this extraction procedure is considered undesirable for large-scale production. In addition, iso-electric focussing is not amenable to large-scale production. A second procedure (U.S. Pat. No. 5,101,014) involved the extraction of the outer membrane protein from the cells of an afimbriated strain of B.pertussis. The protein was purified by a combination of DEAE-Sepharose and Affigel-blue chromatographies. The potential of leaching the blue dye into the product would be a possible safety concern. Neither method addresses the purification of pertactin directly from fermentation broths.